In recent years, thin-film semiconductor devices have been widely used such as thin film transistors (TFTs) and thin-film solar batteries. A TFT includes, for example, a substrate, a gate electrode, a gate insulating film, a semiconductor layer, a source electrode, a drain electrode, a protection layer, and so on. Also, in order to improve carrier mobility in a semiconductor layer, development of TFTs has been proceeded using oxide semiconductor such as amorphous indium gallium zinc oxide (IGZO, InGaZnO).
By the way, when hydrogen intrudes into an oxide semiconductor layer, the oxide semiconductor layer is made to be conductive, and as a result a TFT might not function as a transistor. In response to this, it is effective to form, as a protection layer, a hydrogen protection film constituted from an aluminum oxide film so as to cover the oxide semiconductor layer.
In order to form wirings in a TFT, on the other hand, it is necessary to provide through-holes in the aluminum oxide film. Wet etching with use of an etching solution should be for example performed for providing through-holes in a thin film (Patent Literature 1).